The manufacture of semiconductor devices involves the performance of a series of process steps using a variety of high tech production and metrology tools in a certain order and often within a certain period of time. The primary function of a wafer logistics system in a semiconductor fabrication facility, or “fab,” is to deliver the wafers to each of the tools at the right time, as well as to track the location and status of the wafers throughout the process.
Automated material handling systems (AMHS) are used in fabs to carry out the automated functions more efficiently, consistently, and safely than can be done via manual means. Process and tool changes have placed additional demands on the AMHS. Such demands include the necessity for cross-floor and cross-phase transportation and increased transportation volume, the combination of which often results in traffic jams, delays, and lower tool utilization.
When a wafer carrier, such as a standard mechanical interface (SMIF) pod or front opening unified pod (FOUP) containing wafers, is to be transferred, a manufacturing execution system (MES) determines to what destination in the fab the wafer carrier should be transferred. In an AMHS, stockers are widely used in conjunction with automatically guided or overhead transport vehicles, either on the ground or suspended on tracks, for the storing and transporting of semiconductor wafers in SMIF pods or in FOUPs. Once the destination decision has been made, the MES sends a transfer request to a material control system (MCS), which calculates a detailed transportation route using a real time dispatching (RTD) system and then notifies a transfer manager to execute the transfer step-by-step. AMHS's have been used extensively in the semiconductor fabrication field.